If I understand you correctly, you want to remove everything from the string except for standard characters from the basic Latin alphabet. Is this correct?
If so, you can do this:
<?php
$string = 'Hello, This is a text !';
$plainText = preg_replace('/[^a-z\\s]/i', '', $string);
echo $plainText; //Prints "Hello This is a text "
?>
This will remove everything except for alphabetical characters and whitespace. If you want to allow digits as well, you can replace the third line with this:
Also note that in the example you gave (i.e. “Hello, This is a text !”), you will end up with a space after “text”. If you want to remove the extra whitespace from both sides, you can use this: